Land Cover, Climate and Management

1
Land Cover, Climate and Management in
Rangelands Steve Archer University of Arizona
Earth Systems Feedbacks Vulnerability of Carbon
Cycle to Drought and Fire Canberra,
Australia 7 June 2006
2
Archer et al. 2001
Grassland, Pasture or Savanna
Desert Scrub
Woodland
Grassland or Shrub-Steppe
Grassland or Savanna
Forest or Woodland
DESERTIFICATION
THICKETIZATION
DEFORESTATION
3
1903
In many drylands, woody plants have displaced
grasses in recent history.
1941
New Mexico
Arizona
4
South Dakota
1874
Kansas
1975
5
Causes subject of active debate include
changes in Climatic regimes (amount
and seasonality of rainfall) Herbivory
(increased grazing or decreased browsing)
Fire regimes (decline in frequency,
intensity) Atmospheric CO2 Enrichment
N-deposition These factors have likely
interacted to produce the changes observed to
date
6
Acacia nilotica invasion of Mitchell Grasslands,
QLD, Australia Cannot invoke climate, CO2,
N-deposition as proximate causes for this
change Climate does, however, affect rates
and dynamics of spread and stand development
Photo S. Archer
7
WOODY PLANT ENCROACHMENT INTO GRASSLANDS
SAVANNA (see biblography at http//ag.arizona.ed
u/research/archer/)
LOCATION GENUS REFERENCE ARIZONA Prosopis
Bahre Shelton (1993) Larrea Humphrey
Mehrhoff (1958) CALIFORNIA Baccharis Williams
et al. (1987) Juniperus Young Evans
(1981) KANSAS Quercus, Juniperus Knight et al.
(1994) NEBRASKA Quercus, Juniperus Steuter et
al. (1990) NEW MEXICO Prosopis,
Larrea Buffington Herbel (1965) MONTANA Pseudo
tsuga Arno Gruell (1986) NEVADA Pinus,
Juniperus Blackburn Tueller (1970) NORTH
DAKOTA Pinus Potter Green (1964) OKLAHOMA Juni
perus Snook (1985) SOUTH DAKOTA Pinus Progulske
(1974) Quercus, Celtis Tieszen Archer
(1990) TEXAS Juniperus McPherson Wright
(1990) Prosopis, Other Archer
(1989) Larrea Wondzell Ludwig
(1995) UTAH Pinus, Juniperus Madany West
(1983) Phenomenon also widespread in
rangelands of Africa, South America,and
Australia
8
RATES, DYNAMICS PATTERNS OF WOODY
ENCROACHMENT
Rapid Non-linear punctuated by
climatic events Spatially heterogeneous -
soils - topography Topoedaphic carrying
capacity?
9
Flint Hills, KS
Climate X Grazing
X Fire
Interaction
Mitchell Grasslands, QLD, Australia
10
Archer 1989
Graminoid-driven succession
Woody Plant-driven succession
Perennial grasses
Short / low
Herbaceous retrogression
A Tall / mid-grasses B Mid / short
grasses C Short grass / annuals Transition
threshold
A
B
C
Time or cultural energy increments required to
drive system to new configuration
Community Composition
Shrubland or Woodland Domain
Grassland Domain
Woody plants
Long / high
Time
high low low
low low high
Fire Frequency
Grazing Pressure
high
Probability rate of woody plant establishment
11
Changes in woody plant cover, southern Great
Plains (Archer et al . 1988)
40 30 20 10
Site 1 Site 2 Site 3
Woody Cover ()
1941 1963 1983
Year
12
Traditional Perspectives on Woody Plants in
Drylands Livestock production
forage production animal handling
animal health Wildlife habitat management
(game species) Watershed management
stream flow ground water recharge
Herbaceous Biomass
Woody Plant Abundance
13
Emerging perspectives Biodiversity
Biogeochemistry and land surface-atmosphere
feedbacks owing to changes in N- cycle C-
cycle Water-cycle Non-methane hydrocarbon
fluxes Biophysical properties
14

• BIOGEOCHEMICAL PERSPECTIVE
• Land-based based inventory (Houghton et al.
  1999,
• 2003) and tracer-transport inversion (Pacala
  et al.
• 2001) methods suggest woody plant
  proliferation
• in non-forest systems is significant component
  
• of missing carbon sink in North America
• Accurate estimates of extent of woody plant
• encroachment lacking. Poor understanding of
• effects on C pools/fluxes. Hence, estimates
  highly
• uncertain.

15
Changes in Woody Plant Cover (1950-1990)
La Copita Site, Texas
(Archer et al. 2001)
Landscape 1 (1950)
1976
1950
1990
16
(No Transcript)
17
Change in Soil Carbon with Woody Plant
Proliferation Broad Survey
Jackson et al. 2002
NM Argentina
TX MN
Venezuela Africa
Relative Change ()
Precipitation (mm)
18
Why the variable effects on SOC?
Species Effects Tarbush (Jornada
site) Creosote bush Temperature
Effect Temperate site 9 (Prosopis
communities) Subtropical site 27-103 Soils
Effect Sandy loam soils 27-37 (subtropical
site) Clay loam soils 103 Land Use
History Grazing, Fire, Brush Management,
Erosion
19
Asner et al. 2003
400 km2
1937 1999
Net 30
lt10 gt90
20
(Asner, et al. 2003)
1999
1937
21
ARC
Ongoing intercomparison of sites undergoing woody
plant proliferation
VCR
WYO
KNZ
SEV
VER
JOR
LAC
22
ARC
VCR
WYO
KNZ
SEV
VER
JOR
LAC
23
ARC
WYO
VCR
KNZ
SEV
VER
JOR
LAC
24
ARC
VCR
WYO
KNZ
SEV
VER
JOR
LAC
25
ARC
Compare/contrast changes accompanying WP
increases in ANPP Biomass LAI SOC
VCR
WYO
KNZ
SEV
VER
JOR
LAC
26
Archer, S. 1989. Have southern Texas savannas
been converted to woodlands in recent history?
American Naturalist 134545-561. Archer, S.
2006. Bibliograpy of woody plant encroachment in
drylands. http//ag.arizona.edu/research/archer/)
Archer, S., T. W. Boutton, and K. A. Hibbard.
2001. Trees in grasslands biogeochemical
consequences of woody plant expansion, pp.
115-138. In E.-D. Schulze, M. Heimann, S.
Harrison, E. Holland, J. Lloyd, I. Prentice, and
D. Schimel, eds. Global biogeochemical cycles in
the climate system. Academic Press, San
Diego. Archer, S., C. J. Scifres, C. R. Bassham,
and R. Maggio. 1988. Autogenic succession in a
subtropical savanna conversion of grassland to
thorn woodland. Ecological Monographs
58111-127. Asner, G. P., S. Archer, R. F.
Hughes, R. J. Ansley, and C. A. Wessman. 2003.
Net changes in regional woody vegetation cover
and carbon storage in Texas Drylands, 1937-1999.
Global Change Biology 9316-335. Hibbard, K.,
D. Schimel, S. Archer, D. Ojima, and W. Parton.
2003. Grassland to woodland transitions
integrating changes in landscape structure and
biogeochemistry. Ecological Applications
13911926.